Blood

Dylan seemed a bit brighter this morning after a good night's sleep with me on the sofa next to him. He got up to go toilet and did a big wee... great I thought, but immediately realised that it was dark red, almost black. I wasn't quick enough to get a container to get some, but I used a tissue afterwards to soak some up to show the vet... Why is there so much blood in his urine? I am of course taking him back to the vet's this afternoon when they open...this really has me worried.